Solid-state optically pumped lasers comprise, in general, a solid-state lasing material commonly called the gain medium, disposed between two mirrors which form an optical cavity. An optical pump source such as a laser diode or an array of diodes, generates a pump beam or light wave which is focused onto the gain medium. Energy from the pump laser beam in the form of photons is absorbed by the gain medium, energizing gain medium atoms into an excited energy state. A laser system exhibits a population inversion when atoms in an excited energy state are higher in population than atoms in a ground state. When a photon encounters an energized atom in the gain medium, stimulated emission occurs. The photons enter an entrance mirror at one end of the cavity and travel along the axis of the cavity until they are reflected by an exit mirror and returned to the gain medium. Stimulated emission within the gain medium again amplifies the light beam or wave and eventually an optical standing wave resonates within the optical cavity. With proper design, the wave oscillates in a single axial mode within the laser cavity. With single mode oscillation, there are nulls in the standing wave intensity pattern. At these nulls, the inverted population in the gain medium is not saturated.
Methods have been described for attaining two linearly polarized orthogonal axial modes of propagation from a solid-state laser resulting in an axially uniform intensity pattern:
1. V. Evtuhov & A. E. Siegman, A "Twisted Mode" Technique for Obtaining Uniform Energy Density in a Laser Cavity, Applied Optics, Vol. 4, No. 1, January 1965, at 142. PA0 2. D. W. Anthon et al., Intracavity Doubling of CW Diode-Pumped Nd:YAG Lasers with KTP, IEEE Journal of Quantum Electronics, Vol. 28, No. 4, April 1992, at 1148.
In the Evtuhov & Siegman reference, a pair of birefringent elements, such as quarter-wave plates, are provided, one on each side of the gain medium inside the laser cavity. The quarter-wave plates are oriented such that the fast axis of one plate is oriented 90.degree. to the fast axis of the other. The gain medium is either optically isotropic or oriented so that the axis of the laser cavity is parallel to the optical axis of the medium. The eigenpolarizations, or allowed polarizations, of the cavity are linear and oriented along the wave plate axes. The optical path lengths are identical for the two polarizations since the quarter-wave plates are oriented at 90.degree..
A standing-wave pattern is produced which is axially uniform in intensity, eliminating the nulls in the inverted population. One mode of propagation is orthogonally polarized with respect to the other mode. The sum of the intensities of the two modes is constant along the axis of the cavity. Thus, with two orthogonally polarized modes oscillating in the cavity, the inverted population is uniformly saturated, which precludes other modes from oscillating. Hence, the output of the laser system is a single frequency, dual mode, orthogonally polarized light wave.